Various improved passive optical devices have been utilized in the optical transmission system of today.
An optical circulator is one of such passive devices for use in the optical transmission system. The optical circulator has, for example, four ports for entrance and exit of a light beam. A beam introduced from a first port is transmitted to a second port. A beam introduced from the second port is transmitted to a third port. A beam introduced from the third port is transmitted to a fourth port. A beam introduced from the fourth port is transmitted to the first port. Four ports are optically interconnected so that a beam is cyclically transmitted in the above manner, i.e. 1.fwdarw.2, 2.fwdarw.3, 3.fwdarw.4, 4.fwdarw.1. Such an optical circulator will make it possible to realize a complicated function such as two-way communication, impediment research, or data link, in an optical transmission system in the near future.
Occurrence of transmission loss or crosstalk should be minimized in the optical circulator as well as in an optical switch or an optical branching filter. In order to obviate the transmission loss and the crosstalk, rays of light emitted from an inlet optical fiber are made parallel by means of a lens, so as to make a beam. Then, this beam is operated on in a prescribed manner within the device. After that, the beam is converged to an outlet optical fiber by another lens. However, some light leaks from the beam even though the rays of the beam are paralleled by the lens. Therefore, there is some transmission loss of light between the inlet fiber and the outlet fiber. Accordingly, it is desirable that the size of the optical device be minimized so as to avoid the path length of light being elongated, in order to minimize transmission losses.
In general, a polarizer used in the optical circulator or the optical devices comprises two prisms of anisotropic crystals faced to each other. The prisms are usually made of calcite. An incident beam upon the facing plane is separated into two polarized beams, i.e. a beam of ordinary rays and a beam of extraordinary rays. The polarizations of the beams are perpendicular to each other.
Examples of such a polarizer or a beam splitter in the prior art are disclosed in Japanese Patent Publications 45-13278 and 52-49967 and Laid Open Japanese Patent Application 49-93028. In the polarizer of the prior art, the size of the prism is large when compared with the diameter of the beam. Therefore, it is difficult to make the optical device comprising this polarizer to be small in size. Besides, the beam is not reliably separated in the prior art polarizer. Therefore, when the polarizer is used in an optical circulator, excessive transmission loss and the problem of crosstalk occur.